This invention relates to a machine for knitting a tubular fabric, in particular stocking articles, of the type which comprises a needle cylinder mounted coaxially to a cylinder holder, wherewith it rotates about a common vertical axis, a dial carried above the needle cylinder in a predetermined spaced-apart relationship therewith and set for rotation about said vertical axis, a tubular fabric intake and discharge duct in fluid conducting communication, on one side, with the inside of said cylinder holder, and on the other side with a suction source, and means for powering the cylinder holder. To produce a tubular fabric on a machine of the aforesaid type (commonly referred to as a circular knitting machine), it is mandatory that the dial and needle cylinder rotate in preestablished timed or in-phase relationship to each other and maintain such timing throughout the production or knitting cycle.
Any undesired out-of-time, or out-of-phase, condition, however small, generally results in such plainly visible faults in the fabric as to require discarding of the end product.
In currently available circular knitting machines, the operational rotary movements of the needle cylinder and dial are derived from a common drive shaft. In general, whereas the rotation of the needle cylinder is obtained through a pair of bevel gears, one whereof is keyed to the drive shaft and the other to the cylinder holder, the rotation of the dial is derived from said drive shaft through a drive train having rigid component members and which extends outside the machine and, in most favorable of cases, includes at least three pairs of bevel gears. It will be appreciated that, in spite of a most careful manufacturing of the bevel gears, their meshing unavoidably involves the presence of backlash, which increases with wear as the machine is being operated. In the drive train from the drive shaft to the dial, said backlash of each bevel gear pair adds up in a sort of negative synergism, with a resulting real danger of the dial going out of time or phase with respect to the needle cylinder.
That danger, which grows continuously in time as the bevel gear wear progresses, has been unavoidable heretofore, to the point that a more or less pronounced production waste is normally accepted (albeit forcibly so) in the art. In order to limit the magnitude of said production waste or loss, the experts' attention has concentrated so far upon improvements to the bevel gear manufacturing technology, as well as the meshing thereof, and on an increase of the maintenance frequency, such as to prevent the damage induced by wear. However, whereas on one side, the production loss caused by the out-of-phase relationship between the dial and needle cylinder, due essentially to the aforementioned drive train remains substantial, on the other side, the costs related to the manufacturing and maintenance of the bevel gear pairs and, more generally, of the whole drive train are increased considerably.
Another major drawback is the space requirements which result from the currently adopted arrangement of said drive train externally to the circular knitting machine.
In the German published patent application (German Auslegeschrift) No. 1,103,505, there is disclosed a circular knitting machine of large diameter, wherein the operational rotary movement of the dial is derived from the motion of the cylinder holder through a drive train which extends inside of the cylinder holder; in that machine, the resulting tubular product is cut, as it is being knitted, along a generatrix thereof and discharged from the machine in the form of a flat fabric. That lengthwise cutting operation is a strict necessity, according to the teachings of the German patent, as otherwise it would not be possible to discharge the tubular fabric, because owing to its encircling the drive train right from the start, it would be withheld by the drive train itself, and get caught and entangled within the machine.